Motor vehicle fuel tanks typically comprise a metal or synthetic material tank wall and a fuel sender assembly (also called a fuel pick-up assembly) mounted within the fuel tank and adapted to provide fuel to the vehicle engine. Generally the amount of fuel supplied to the engine is controlled by a carburetor which typically includes a carburetor bowl, that is a fuel reservoir within the carburetor. Such fuel reservoir is necessary since, if the fuel level in the fuel tank is low, the supply of fuel by the fuel sender assembly may be intermittently interrupted. This is especially so during vehicle acceleration, turning, etc. More recently, vehicle engines have employed electronic fuel injection systems which have no carburetor bowl fuel reservoir. Accordingly, it is necessary to provide a reservoir in the fuel tank to insure continuous fuel supply by the in-tank fuel sender assembly during low fuel conditions. If fuel is not available to the fuel sender assembly, engine stalling will result. Numerous in-tank reservoir designs are known to the art. Typically, however, these designs involve multi-component assemblies which involve costly assembly operations and undesirable added weight. U.S. Pat. No. 4,305,416 to Henning et al is typical of such multi-component assemblies. It suggest a fuel tank having a back-up tank assembly and related components mounted within the fuel tank.
Previously, unsuccessful attempts have been made to fluid pressure thermoform a fuel tank wall having a molded-in reservoir in the bottom surface of the tank, that is, to provide a convoluted bottom wall during thermoforming thereof to form a fuel reservoir. As used herein, fluid pressure thermoforming refers to any technique in which air (or other gas or liquid) pressure against the surface of a heated thermoplastic work piece is used to conform the work piece to the configuration of an underlying forming tool. Exemplary of well known fluid pressure thermoforming techniques is blow molding, in which a parison of molten thermoplastic material is formed and enclosed within a molding chamber. The ends of the parison are sealed and fluid pressure is injected into the parison to expand it outwardly against the surface of the molding chamber. Another exemplary fluid pressure thermoforming technique is vacuum forming, in which vacuum is applied from the surface of a molding tool causing atmospheric pressure to conform the work piece to the surface of the molding tool. Motor vehicle fuel tanks formed by such fluid pressure thermoforming techniques can provide significant cost and weight advantages and design flexibility advantages over fuel tanks formed of other materials and by other methods. It previously has not been possible, however, to fluid pressure thermoform a weight-competitive fuel tank with a suitably effective unitary fuel reservoir in a cost-effective manner without unacceptably thin tank wall areas (due to stretching of the wall material to form the reservoir) and other unacceptable structural deficiencies. It is an object of the present invention to provide a fluid pressure thermoformed fuel tank comprising a unitary fluid reservoir in the tank wall.